The present invention relates to a printed circuit board fixture assembly and more particularly relates to a fixture for temporary securement of a printed circuit board during processing such as during wave soldering. More particularly, the present invention relates to a fixture assembly from multiple plates or substrates for masking a circuit board so that the leads of components are exposed while surface mounted components (SMTs), are protected or masked from the solder.
Printed circuit board assemblies (PCBA""s) often carry surface mount technology components referred to as xe2x80x9cSMTsxe2x80x9d which are mounted and soldered to one or both sides of the board. The same printed circuit board assemblies also may contain plated through hole components (PTH""s) on one or both sides of the PCB. During the wave the soldering procedure it is conventional practice to place a mask of some type over the surface on which soldering of the leads is to occur. The mask has openings which expose one or a plurality of the leads to be soldered. The mask will typically have recesses which receive the surface mounted components on the downwardly disposed surface of the board and shield these components during the wave soldering procedure.
Various devices for masking circuit boards during wave soldering may be found in the prior art. U.S. Pat. No. 5,454,505 describes an apparatus for facilitating wave soldering treatment and other handling of circuit components during manufacture. The apparatus has a rigid frame symmetrical about a plane and defining an opening which traverses the frame intermediate a first side and second side. The apparatus further comprises a masking member affixed to the frame defining a plurality of apertures traversing the masking member to the opening in the frame. A holding device maintains the circuit components in an operational location in fixed relation with the masking member with the frame during wave soldering. Areas in the circuit component are substantially in registry with an aperture of the masking member when the circuit component is in the operational location. The walls about the apertures have a ramp for effecting solder flow during wave solder treatment.
U.S. Pat. No. 4,739,919 discloses a soldering mask for soldering the leads of leaded components to a circuit pattern on a surface of the circuit board having surface mounted components already soldered to the circuit pattern on that surface. The mask comprises a flat metal plate having a recess in the top surface with a plate for reception of the circuit board. Cavities in the bottom surface of the recess house the surface mounted components. Openings through the plate from the bottom surface of the recess permit passage of leads and also permit access by the solder to the solder leads of the circuit pattern.
While the above patents and other similar devices in the prior art facilitate masking of circuit boards for wave soldering, there nevertheless exists a need for a fixture which can be economically manufactured without the necessary expensive machining and milling operations. There further exists a need for a more versatile, less expensive fixture having components which can be used with various PCBA""s.
Briefly, the present invention provides a fixture or holding assembly that is assembled from a plurality of individual substrates or plates that have been fabricated to accommodate the requirements of a particular PCBA. The first or top plate is configured having a cut-out to conform to the perimeter of the PCBA and to properly align the PCBA to be processed. An intermediate or middle plate has cutouts of the various sizes and shapes to accommodate various SMT devices that are mounted on the surface of the PCBA that will be downwardly disposed during wave soldering. This downwardly disposed surface of the PCBA is typically referred to as the secondary side. The intermediate plate will contain cutouts of various sizes which become protective cavities once the bottom or lower plate is attached to the bottom or secondary side of the intermediate plate. The intermediate plate may also contain apertures of various sizes to accommodate the leads of PTH devices mounted on the upper surface of the PCBA which is typically referred to as the primary side. These cutouts will align and register with similar apertures in the bottom most plate to allow leads to pass through both the intermediate and bottom plates.
The intermediate plate is of the appropriate thickness to accommodate the height of the SMT and PTH devices mounted on the secondary side so that when the bottom plate is attached, the cut-outs in the intermediate plate define protective cavities and recesses. The bottom plate is secured to the bottom surface of the intermediate plate and has apertures of various shapes that align with similar cut-outs to accommodate the passage of the PTH devices. Since the lower plate defines apertures only for the leads of the PTH devices on the primary side of the PCBA, the addition of the bottom plate shields and protects the bodies of the SMT and PTH devices that are mounted on the secondary side of the PCBA. An additional, optional plate termed a xe2x80x9cweighting platexe2x80x9d may also be provided which will hold the PCBA flat against the surface of the intermediate plate. The optional weighting plate has locating holes which registers with alignment pins or posts. The weighting plate has adjustable fasteners inserted to appropriate depths so that the weighting plate applies a downward force against the PCBA through the depending fasteners. Preferably the fasteners are threaded members of nylon or similar material which will contact the PCBA only in unpopulated areas.